The UMP trigger group uses a sear as a secondary safety (even on the FBI or semi-only trigger groups). Now the area that the sear would fit into in the USC receiver is blocked, and the ATF has stated that cutting that area open is manufacturing a machine gun (big no-no). (since the sear is also used for the auto function on the Navy and SEF trigger groups)

If you take the sear out then you can use the trigger group fine, but you are missing a safety component.

The USC trigger group uses a different style of secondary safety (that also lessens the trigger pull weight) and doesn't require the cut. Since the USC and UMP use the same pin locations (except for the sear pin which doesn't get used by the USC parts) you can just swap the parts the retain all the safety features of the USC trigger parts!

The UMP trigger group uses a sear as a secondary safety (even on the FBI or semi-only trigger groups). Now the area that the sear would fit into in the USC receiver is blocked, and the ATF has stated that cutting that area open is manufacturing a machine gun (big no-no). (since the sear is also used for the auto function on the Navy and SEF trigger groups)

If you take the sear out then you can use the trigger group fine, but you are missing a safety component.

The USC trigger group uses a different style of secondary safety (that also lessens the trigger pull weight) and doesn't require the cut. Since the USC and UMP use the same pin locations (except for the sear pin which doesn't get used by the USC parts) you can just swap the parts the retain all the safety features of the USC trigger parts!

The UMP trigger group uses a sear as a secondary safety (even on the FBI or semi-only trigger groups). Now the area that the sear would fit into in the USC receiver is blocked, and the ATF has stated that cutting that area open is manufacturing a machine gun (big no-no). (since the sear is also used for the auto function on the Navy and SEF trigger groups)

If you take the sear out then you can use the trigger group fine, but you are missing a safety component.

The USC trigger group uses a different style of secondary safety (that also lessens the trigger pull weight) and doesn't require the cut. Since the USC and UMP use the same pin locations (except for the sear pin which doesn't get used by the USC parts) you can just swap the parts the retain all the safety features of the USC trigger parts!

Both the UMP and the USC use the same Sear.

The UMP has a Release Lever .

The "secondary safety" is really not a safety at all. It relieves tension of the Hammer spring for a minutely lighter trigger pull but it does not prevent the hammer from accidentally falling.

I don't believe that the UMP or USC or even SL-8 trigger parts are garbage, they are military grade. The parts are light weight, made up of composite materials and work well together. The springs are heavy to fire military grade primers. There are large lips on the sear catch areas of the hammer for long life and durability. But this is a military grade gun and the heavy trigger is the secondary safety. You have to want to pull the trigger and not accidentally pull the trigger.

I think the long story short is that it "looks" better to the ATF if you mostly have a USC with a few UMP parts for cosmetic reasons than if you have mostly a UMP with some USC parts.

Originally Posted by singtoe

The "secondary safety" is really not a safety at all. It relieves tension of the Hammer spring for a minutely lighter trigger pull but it does not prevent the hammer from accidentally falling.

X2 this has been discussed ad nauseum and it is not any kind of secondary safety as is commonly believed. If anyone can show how it can act to prevent the hammer from falling in the event of something breaking I'll put up a G36 mag as a reward. The confusion is understandable since it LOOKS like it should stop the hammer.
I think we also determined that it might have some functionality in the two round burst pack, Chop, am I remembering correctly?

The "secondary safety" is really not a safety at all. It relieves tension of the Hammer spring for a minutely lighter trigger pull but it does not prevent the hammer from accidentally falling.

I don't believe that the UMP or USC or even SL-8 trigger parts are garbage, they are military grade. The parts are light weight, made up of composite materials and work well together. The springs are heavy to fire military grade primers. There are large lips on the sear catch areas of the hammer for long life and durability. But this is a military grade gun and the heavy trigger is the secondary safety. You have to want to pull the trigger and not accidentally pull the trigger.

By sear I meant the catch that locks the hammer back until the bolt goes all the way forwards. As far as I know both the USC and UMP lowers use the same trigger and disconnector, and yes (as seven.sixtwo pointed out above me) what HK lists as the "secondary safety" doesn't actually hold the hammer down at all, it just lightens the trigger pull a bit, but as far as trigger parts though, there is no reason not to have it (as if you just took the "sear" or release lever out from the front area so the group can fit into the USC upper)

I think the confusion comes from it being listed as "secondary safety" in the HK parts list.

By sear I meant the catch that locks the hammer back until the bolt goes all the way forwards. As far as I know both the USC and UMP lowers use the same trigger and disconnector, and yes (as seven.sixtwo pointed out above me) what HK lists as the "secondary safety" doesn't actually hold the hammer down at all, it just lightens the trigger pull a bit, but as far as trigger parts though, there is no reason not to have it (as if you just took the "sear" or release lever out from the front area so the group can fit into the USC upper)

I think the confusion comes from it being listed as "secondary safety" in the HK parts list.

The USC or the UMP does not have a disconnecter. As for the "secondary safety", I looked all over the HK Manual and it never lists it as a secondary Safety. They list it as a Locking Lever, Locking Lever Spring and Locking Lever Housing.

In a firearm, the sear is the part of the trigger mechanism which holds the hammer or striker back until the correct amount of pressure has been applied to the trigger; at which point the hammer or striker is released to discharge the weapon. The sear may be a separate part or can be a surface incorporated into the trigger, such as the AK47. This by definition.

I think the long story short is that it "looks" better to the ATF if you mostly have a USC with a few UMP parts for cosmetic reasons than if you have mostly a UMP with some USC parts.

X2 this has been discussed ad nauseum and it is not any kind of secondary safety as is commonly believed. If anyone can show how it can act to prevent the hammer from falling in the event of something breaking I'll put up a G36 mag as a reward. The confusion is understandable since it LOOKS like it should stop the hammer.
I think we also determined that it might have some functionality in the two round burst pack, Chop, am I remembering correctly?

Don't bother asking HK USA they really don't like that topic. I am locked out of the customer service forum.